Indoor radio systems have been used since the first generation of cellular systems to compensate for coverage holes where the outdoor macro network cannot reach the users located inside buildings.
Today, indoor deployments are becoming increasingly popular since the macro networks are capacity limited as a result of the exponential traffic growth, or the outdoor cells do not provide the needed coverage for the requested bitrates. That is, even if users inside a building can be reached, the macro cell (or cells) covering the building may be limited in capacity, so to provide the promised capacity an indoor radio system must be installed.
FIG. 1 is a schematic diagram illustrating an example of a wireless communication context including an outdoor domain and an indoor domain in which wireless communication devices 30 are located. An indoor radio system 10 is responsible for serving wireless communication devices 30 in the indoor domain using one or more radio heads 15. A base station 20 is configured to provide one or more macro cells for serving wireless communication devices 30 in the outdoor domain, and possibly also in the indoor domain. The indoor radio head coverage may also extend to the outdoor domain, which will be discussed later on.
There exists a variety of indoor radio systems, also sometimes referred to as In-Building Systems, IBS, for deployment of antennas indoor.
Distributed Antenna Systems, DAS, have been the most common solution. In an active DAS, the antenna points include means for amplifying the signal, and in some cases also for translating the signal to a different frequency. This frequency can be both lower and higher than the original frequency. In an active DAS based on fiber-optical repeaters, the signal is transformed into an optical signal that is transmitted over low loss optical fibers to a central point where the signal is converted back to Radio Frequency, RF, and subsequently digitalized and down-converted to baseband. RF signals from multiple antenna points are, typically passively, combined at the central point.
Another solution, introduced by the Ericsson Radio Dot System, RDS, involves down-converting the signal to Intermediate Frequency, IF, and transmitting with low loss over CAT cables. (Passive) combination of signals from multiple antenna points can be done on IF. Common for all active DAS implementations is that it might be possible to tag the signals received from different antenna points with information identifying which antenna point it is transmitted (received) from. Different methods for doing so are known in the art.
The RDS system is in one perspective a complete radio base station with split architecture, but can also be seen as one type of active DAS, with integrated antenna(s) in the remote unit.
FIG. 2 is a schematic diagram illustrating a non-limiting example of an indoor radio system. In this particular example, the indoor radio system 10 comprises a Base Band Unit, BBU, 11 and Indoor Radio Unit, IRU, 13, and one or more radio heads 15 having one or more associated antennas. The radio head 15 or dot is the remote unit with integrated antenna. In a RDS system, the IRU is the head end interfacing the radio head or dot over CAT cable. The IRU may be connected to the BBU, through a CPRI interface.
FIG. 3 is a schematic diagram illustrating an example of the coverage area of an indoor radio system having multiple radio heads. In this example, the overall radio coverage of an indoor radio system is built up from the radio head coverage of multiple radio heads 15-1 to 15-5.
When deploying an indoor radio system, such as an RDS, the most common goal is to cover all users located inside the building, and no users outside of the building. This can be achieved through a combination of thorough pre-planning of system deployment followed by reconfiguration of system parameters based on measurements/tests in and around the building after the installation is completed. Such planning and, especially, testing and tuning is however time consuming and costly, and most of the time beyond the budget for such an installation. A normal installation consists of a rough dimensioning followed by a quick verification of the performance inside the building. To avoid having to reconfigure the system, the system is often over-dimensioned in terms of output power and node density, leading to a coverage area including also the outdoor surroundings of the building.
Radio coverage outside the building can sometimes result in unwanted behavior and negative consequences. By way of example, if a street is covered, cars passing by may handover to the RDS system and then back to the macro network. Handover can have impact on user quality experience and even lead to dropped calls. Also, positioning of emergency calls may faulty indicate the call being inside the building instead of outside the building.
Reference [1] relates to setting a transmit power of a femto cell depending on received power of at least one neighboring macro cell.